a) Field of the Invention
The present invention relates to a lens system with a magnification changing function, having a low-pass filter. More specifically, it relates to a small size and inexpensive zoom lens or lens system for cameras using electronic image pickup means such as a camcorder or a digital camera, and a camera using such a zoom lens or lens system.
b) Description of Related Art
Recently, a small-sized and inexpensive zoom lens for cameras using an electronic image pickup means, such as a camcorder and a digital camera, is required particularly for civilian use.
An approach for coping with the matter is to review the powers of movable units in a zoom lens system or the moving mode of moving units at the time of zooming. Solutions accordingly obtained include reduction of the number of lens elements.
For example, as a conventional zoom lens system, that comprising, in order from an object side, a first lens unit with a positive power, a second lens unit with a negative power, an aperture diaphragm, a third lens unit with a positive power, and a fourth lens unit with a positive power, wherein the second lens unit is moved for changing magnification and the fourth unit is slightly moved for image plane compensation and focusing, is known. In this type of system, since the image pickup surface side of the aperture diaphragm is not provided with a magnification changing function, the position and the size of the exit pupil do not change, the F-number or the incident angle on the image pickup surface is not subject to a change during magnification change, and thus the configuration can be provided comparatively easily with a small number of moving units. However, in terms of downsizing, since the second lens unit is burdened with substantially most of the magnification changing function, a problem arises in a large moving amount of the lens unit, and a large total length.
On the other hand, Japanese Patent Application Preliminary Publication Nos. Hei 6-94997 and Hei 6-19457 propose a method for downsizing by giving a magnification changing function to the third lens unit in addition to the second lens unit so that the third lens unit is moved at the time of magnification change. According to this type of proposal, since a lens unit having a magnification changing function is disposed on the image pickup side of the aperture diaphragm, the exit pupil changes accordingly. Therefore, the F-number changes according to magnification change. However, it would not give rise to an essential problem in terms of the photographic function.
Owing to the review of the power and the moving mode of the lens units, design of a new configuration of lens elements and proposal of a smaller optical system can be made.
Another approach is to review the optical low-pass filter used for a camcorder, a digital camera or the like. In photographing using an image pickup device such as a CCD, if frequency components higher than the picture element frequency of the image pickup device are included in the image of the object, the image pickup device generates a false signal or a false color signal so that moire fringes or the like appear in the image. As a countermeasure therefor, it is known that frequency components higher than a predetermined value can be excluded from the image of the object by disposing in the optical system an optical element having the function of an optical low-pass filter. Conventionally, quartz plates have been used as low-pass filters. That is, an object image is separated into two or more in the polarization directions of the light due to the birefringence of the quartz plate so that high frequency components of an object image can be cut off. It is also known that at least two quartz plates are required for obtaining a sufficient low-pass effect.
The quartz plates are advantageous in that a predetermined low-pass effect can be provided regardless of the diaphragm stop, and the deterioration amount of low frequency components, which determines the image quality, is small. However, since they need to be disposed between the lens system and the image pickup device and to have considerable thickness, they are an obstacle to a small-sized apparatus. Moreover, they are disadvantageous for their high cost.
As a low-pass filter other than a quartz plate, a filter that differently acts on light wavefront depending on the portion of the filter through which light is transmitted (hereinafter referred to as a low-pass phase filter) is known. For example, a phase filter proposed by Japanese Patent No. Sho 44-1155 is advantageous for having a thinner thickness compared with a quartz plate. However, there have been pointed out problems such that an image of an object not particularly focused on is formed too sharp because the filter merely divides the wavefront. A proposal to solve these problems is made in Japanese Patent Application Preliminary Publication Nos. Hei 10-82974 and Hei 10-104552.
According to the proposal, a) with the phase of the wavefront at the center of an incident beam of light being given as a reference, a step-like region having a phase advancing function to advance the phase of the wavefront of the incident beam of light and a step-like region having a phase delaying function to delay the phase of the wavefront of the incident beam of light are alternately formed in the filter; b) the filter is disposed in the vicinity of the diaphragm; so that low-pass effect is effectively obtained. According to the proposal, advantages of the low-pass filter include unchangeable low-pass effect even in the stopped-down condition, a lower cost than that of quartz plates, and capability of providing the low-pass effect to a polarizing substance, which can hardly be handled with a low-pass filter comprising a quartz plate. In addition to that, a large number of other low-pass phase filters using the diffraction phenomenon, such as a diffraction grating have been proposed.
How to arrange a low-pass phase filter in a zoom lens system is discussed in Japanese Patent Application Preliminary Publication No. Sho 63-287922, for example. Specifically, it discusses, with respect to a two-unit zoom lens system having a first lens unit with a negative power, a second lens unit with a positive power, and an aperture diaphragm disposed between the first lens unit and the second lens unit, the position of a low-pass phase filter, and concludes that disposing the low-pass filter before the second lens unit and moving it integrally with the second lens unit is advantageous. A configuration where a low-pass filter is disposed between a lens system and an image pickup device does not cause change of low-pass effect in accordance with change of magnification, but requires such a low-pass phase filter as is difficult to design and manufacture. Therefore, the low-pass phase filter is disposed between the first lens unit and the second lens unit similar to the aperture diaphragm, and is moved integrally with the second lens unit so as to make the change of the low-pass effect by change of magnification relatively small. However, the proposal is directed to an optical system with a low magnification ratio of 1.68. The problem of change of the low-pass effect still remains, and thus the proposal is not a comprehensive solution. (It is anticipated that the change of the low-pass effect will be larger with a larger magnification ratio).
Moreover, although it is not used as a low-pass filter, a liquid crystal lens, which has a uniform lens effect regardless of the polarization direction and a refracting power that is changeable by application of voltage, is disclosed by Japanese Patent Application Preliminary Publication No. Hei 10-73758.
If a low-pass phase filter is adopted in a zoom optical system with a changing exit pupil for the purpose of downsizing the optical system, since the space for a conventional quartz low-pass filter is not required, further downsizing or a higher magnification ratio can be attained. However, if it is disposed in the vicinity of the diaphragm of the optical system with the changing exit pupil, the low-pass effect is changed in accordance with magnification change and thus an optimum low-pass effect cannot be obtained from the wide angle end to the telephotographic end.